1. Field of the Invention
The present invention relates to a micro-droplet ejection apparatus having nozzle arrays without individual chambers and an ejection method of droplets thereof. More particularly, the present invention relates to a micro-droplet generating apparatus with high nozzle density and a method for ejecting micro-droplets.
2. Description of the Related Art
Micro-droplet ejection apparatuses are widely applied in inkjet printheads of inkjet printers. In addition, the micro-droplet ejectors can also be applied in other technical fields, for example, fuel injection systems, cell classification, pharmaceutical release systems, reagent distribution on biochips, direct jet printing photolithography, and micro-injection propelling systems. The common point of all of the above-mentioned applications is that a reliable micro-droplet ejection apparatus with low cost, high frequency, and high resolution is required.
Recently, among the known and used micro-droplet ejection apparatuses, only a few kinds of the ejection apparatuses have been able to individually eject micro-liquid drops with identical shapes. The method of ejecting the droplets with thermally driven bubbles is advantageous as it is relatively simple and the manufacturing cost is relatively low.
The disadvantages of the thermally driven bubble system (also referred to as the bubble injection system) are the problems of cross talk and satellite droplets. The bubble ejection system uses a current pulse to heat electrodes, thereby vaporizing the liquid in a fluid cavity. When the liquid is vaporized, a bubble is formed on the electrode surface and in the liquid, and expands outwards. The bubble is equivalent to a pump that ejects the liquid into the fluid cavity from a micro-nozzle orifice to form a liquid column, and to finally form a flying droplet.
When the current pulse ends, the bubble shrinks accordingly, and the liquid refills the fluid cavity through capillary tension at the same time. However, the fluid cavities corresponding to the micro-nozzle orifices are isolated by spacers, resulting in flow resistance when the liquid refills the liquid cavities. That is, the speed of the liquid refilling the liquid cavities is reduced, so the frequency of the continuous ejection of the droplets is lowered substantially. If the length of the spacers between the liquid cavities is reduced, the problems of the cross talk and the over refilling between the neighboring liquid cavities may occur.
FIG. 1 is a perspective view of a part of a micro-droplet ejection apparatus of U.S. Pat. No. 6,102,530. FIG. 1 shows a row of nozzles 10 in the micro-droplet ejection apparatus, including a plurality of fluid cavities 14, a manifold 16, a plurality of nozzles 18, a plurality of first heaters 11, and a plurality of second heaters 12. The space of each fluid cavity 14 is formed on a silicon substrate 13, and the fluid cavities 14 are spaced by spacers. Therefore, the density of the nozzles 18 in a unit area is obviously limited by the distance between the fluid cavities 14. If the distance between the fluid cavities 14 is inappropriately reduced, it is liable to induce the cross talk. On the other hand, the length of the fluid cavities 14 is relevant to the flow resistance, and the speed of the liquid 16 refilling the liquid cavities 14 is also affected by the flow resistance.
In view of the above, currently, a micro-droplet ejection apparatus with high frequency and high resolution is required, which not only solves the problems of the cross talk and the slowdown of the refilling of the liquid, but also increases the quantity of the nozzles in one unit area.